1. Field of the Invention
The invention generally relates to a method of controlling particulates generated on the surface of a gas diffuser used during plasma enhanced chemical vapor deposition (PECVD) of a thin film of the kind generally known in the semiconductor industry.
2. Brief Description of the Background Art
The presence of information in this section is not an admission that such information is prior art with respect to the invention described and claimed herein.
Current interest in thin film transistor (TFT) arrays is particularly high because these devices are used in liquid crystal active matrix displays (LCDs) of the kind often employed for computer and television flat panels. The liquid crystal active matrix displays may also contain light-emitting diodes (LEDs) for back lighting. As an alternative to LCD displays, organic light-emitting diodes (OLEDs) have also been used for active matrix displays, and these organic light-emitting diodes require TFTs for addressing the activity of the displays. Solar cells are also of particular interest at this time, due to the high cost of traditional energy sources. The technology used to produce solar cells is very similar to that used to create flat panel displays. Photo diodes in general are produced using the technology which is used to create flat panel displays and solar cells.
By way of example, the thin films which make up a TFT are generally produced using plasma enhanced chemical vapor deposition (PECVD). PECVD employs the introduction of a precursor gas or gas mixture into a vacuum chamber that contains a substrate. The precursor gas or gas mixture is typically directed downwardly through a distribution plate situated adjacent to a substrate on which a film is to be deposited. The precursor gas or gas mixture in the chamber is energized (e.g., excited) into a plasma by applying energy to the gas mixture. The plasma comes into contact with various surfaces within the processing chamber in which the PECVD is carried out, such as: The plasma source gas distribution plate; the susceptor on which a substrate typically rests; the shadow frame used to control build up of deposited film near the edge of the substrate; the chamber liner present adjacent to the plasma formation area within the chamber; and, in the slit valve cavity/opening (where the slit valve is the opening through which a substrate passes when entering and leaving the processing chamber) by way of example and not by way of limitation.
One commonly used method of energy application (by way of example and not by way of limitation) is the introduction of radio frequency (RF) power into the chamber from one or more RF sources coupled to the chamber. The excited gas or gas mixture reacts in the processing chamber and at the substrate surface to form a layer of material on the substrate surface. Typically the back side of the substrate is positioned on a temperature controlled substrate support pedestal, which is typically a susceptor. Volatile by-products produced during the film-forming reaction are pumped from the chamber through an exhaust system.
By way of example, the TFT arrays created using PECVD are typically created on a flat substrate. The substrate may be a semiconductor substrate, or may be a transparent substrate, such as a glass, quartz, sapphire, or a clear plastic film. TFT arrays typically employ silicon-containing films, such as microcrystalline silicon (μc-Si), or amorphous silicon (α-silicon), polycrystalline silicon (polysilicon), n-type (n+) or p-type (p+) doped polycyrstalline silicon, silicon oxide, silicon oxynitride, or silicon nitride. The initial substrate upon which the layered film structure is deposited may vary substantially and may be selected from glass, quartz, sapphire, plastic, or a semiconductor substrate, by way of example and not by way of limitation. The films are typically deposited using a PECVD system or other conventional methods known in the art. During PECVD thin film deposition, some film formation may occur upon various surfaces within the processing chamber, such as the gas diffuser, the susceptor, the shadow frame, the slit valve cavities, and interior liners of the processing chamber.
Problem particulates have been generated during the PECVD deposition of silicon-comprising films (and other thin film layers as well). Due to the nanometer sized features of today's semiconductor devices, the presence of particulates on device surfaces substantially reduces the yield of operable devices produced on a semiconductor substrate. The particulate problem is particularly important when the device surface is of the size used in flat panel displays where the inoperability of contaminated devices in the area of the particulates produces a defect which is a readily apparent source of distraction to the user of display device. Defects on photodiode surfaces used in small device displays and indicators is also a major problem. While defects on solar cell surfaces may not be as critical, the overall performance of the solar cell may be affected if the contaminant level is sufficiently high.
The substrate for a display device employing a TFT structure typically comprises a material that is essentially optically transparent in the visible spectrum, such as glass, quartz, sapphire, or a clear plastic, as previously mentioned. The substrate may be of varying shapes or dimensions. Typically, for TFT applications, the substrate is a glass substrate with a surface area greater than about 500 cm2. A surface area of greater than about 45,000 cm2 is not uncommon. As the size of flat panel displays increase, it becomes increasingly difficult to control particulate generation during the thin film deposition processes.
During investigative studies related to the source of particulates generated during the PECVD film deposition process, it became apparent that a substantial number of particulates which end up on the surface of a TFT device are generated at the surface of the gas diffuser used to supply the reactive gases used to generate films on the TFT structure. FIG. 1 shows a gas diffuser 100 of the kind frequently used in the semiconductor industry during PECVD of thin films on a flat panel display substrate. The gas diffuser is commonly fabricated from an aluminum alloy. Due to the reactivity of gaseous precursors used in the PECVD process for thin film generation of doped or un-doped (intrinsic) amorphous silicon (a-Si), silicon dioxide (SiO2), silicon oxynitride (SiON) and silicon nitride (SiN) films of the kind used in liquid crystal displays (or flat panels), for example and not by way of limitation, it is important to provide a surface on the gas diffuser which is as resistant as possible to chemical reactions which generate particulates. In addition, it is important that there be adequate surface area on the surface of the gas diffuser which faces the TFT substrate, so that residue films generated during the TFT film forming process can adhere to the surface of the gas diffuser rather than fall onto the surface of the TFT substrate. There have been a number of theories about not only the source of particulates, but also methods of preventing particulates from leaving the surface of the diffuser to fall upon a substrate which is processed beneath the gas diffuser.
In the past, in an attempt to protect the aluminum alloy surface from corrosion by the reactive PECVD environment, a layer of aluminum oxide, typically produced by an anodization process, was generated on the surface of the gas diffuser. However, due to the relatively sharp corner radii of the gas-supplying openings on the surface of the gas diffuser, it is very difficult to generate an anodized coating which exhibits sufficient integrity at such sharp corner radii. FIG. 1 shows a schematic of a typical gas diffuser 100 of the kind used in the fabrication of flat screen displays. The gas diffuser 100 is attached to a hoisting device 105 which is used to position gas diffuser 100 in a PECVD processing chamber. The exterior surface 102 of gas diffuser 100 is positioned so that it is facing a substrate (not shown) on which thin films are PECVD deposited. There are thousands of gas-supplying openings 104 on the exterior surface 102 of gas diffuser 100.
FIG. 2A shows a schematic of a gas opening 200 of a kind which may be used as a gas-supply opening 104 on exterior surface 102 of the gas diffuser 100 illustrated in FIG. 1. The flat surface 202 forms the exterior surface 102 of gas diffuser 100, which faces the workpiece substrate upon which a thin film is PECVD deposited. The inside corner radius 214 between flat surface 202 and the diffuser hole surface 204 is a relatively sharp radius. Relative dimensions of the diffuser hole surface 204, the diffuser hole taper 206, the pin hole 208, and the back side hole 210 of the gas opening 200 permit control over gas flow rates during PECVD thin film deposition, as described in the related applications previously referred to herein.
FIG. 2B shows a photomicrograph of a corner radius 214 of the kind shown in FIG. 2A, where the corner radius 214 is located between flat surface 202 of the gas diffuser and the hole surface 204. An anodized layer 222 has been created over the hole surface 204 for purposes of protecting exterior surface of the gas diffuser. However, the anodized layer 222 integrity at a relatively sharp corner radius 214 cannot be maintained, and eventually fails as illustrated at 224 in FIG. 2B.
Just recently we determined that not only does failure of the anodized layer 222 expose the underlying aluminum flat surface 202 to attack by reactive plasma gases, but the anodized layer 222 itself flakes off and adds to the particulate formation problem. Analysis of the composition of the anodized layers which have been in service on the gas diffuser surface for a time period shows a higher fluorine content at the upper surface of the anodized layer, where the anodized layer has pitted and is being attacked by process gases during the PECVD film deposition process. As a result, it was determined that it is advisable not to anodize the aluminum surface of the diffuser.
The non-anodized, bare, polished surface of the aluminum/aluminum alloy gas diffuser continues to be exposed to the harsh environment in the PECVD deposition chamber and is under attack by the PECVD precursor gases and byproducts of the film-forming reactions. This non-anodized, bare, polished surface of the aluminum/aluminum alloy gas diffuser needs to be protected in the best manner possible to reduce the formation of particulates which may fall upon a substrate processed beneath the gas diffuser.